Study: 75 percent of Warming Due to Human Activity

A new analysis of the climate of the last 1,000 years suggests that human
activity is the dominant force behind the sharp global warming trend seen in the
20th century.

The study, by Dr. Thomas J. Crowley, a geologist at Texas A&M University,
found that natural factors, like fluctuations in sunshine or volcanic activity,
were powerful influences on temperatures in past centuries. But he found that
they account for only 25 percent of the warming since 1900. The lion's share, he
said, can be attributed to human influences, particularly to rising levels of
carbon dioxide and other heat-trapping "greenhouse gases" that come from the
burning of fuels and forests.

"These twin lines of evidence provide further support for the idea that the
greenhouse effect is already here," Dr. Crowley wrote in describing the work in
today's issue of the journal Science. Several climate experts said his
findings offer the most direct link yet between people and the 1.1 degree rise
in average global temperature over the last 100 years.

The study has already sparked debate among camps of scientists who dispute
the climate records used in Dr. Crowley's analysis and others who say the oceans
play an underappreciated role in controlling warming and cooling of the planet.

But many climatologists said it marked a substantial move forward in
understanding forces that have warmed the earth and are likely to continue
warming it.

Resolving this puzzle -- the balance between human and natural influences --
has been something of a holy grail in atmospheric science, particularly because
the answer could determine whether countries enact plans in coming years to
reduce emissions of greenhouse gases.

Many scientists said that Dr. Crowley's work, while not definitive, was
helping build a strong case.

"This seems to reinforce the notion that we're into a new mode beyond simple
natural forcing, that something else is taking place," said Dr. Raymond S.
Bradley, the director of the Climate System Research Center at the University of
Massachusetts, Amherst.

"The logical conclusion is there is some role played" by human affairs, added
Dr. Bradley, whose studies of past climates were one of the yardsticks used to
measure the reliability of the new work.

There has been a slow shift toward this conclusion in the last few years. In
April, for example, the United Nations Intergovernmental Panel on Climate
Change, the international group whose work has largely shaped the debate on the
issue for a decade, circulated a draft of its next report on global warming,
saying "there has been a discernable human influence on global climate."

Scientists said the main value Dr. Crowley's study was his specific
description of the limited role of natural forces in recent decades.

In a separate commentary, also published in Science, Michael E. Mann,
a climate expert at the University of Virginia and a research partner with Dr.
Bradley, said the new work also indirectly confirms that computer models that
predict a continuation of the warming in coming decades are reasonably reliable.

The news cheered environmental officials at the White House who have backed
action to control greenhouse emissions, particularly because diplomats will meet
again this fall to debate ways to put into practice the provisions of the Kyoto
Protocol, a 1997 agreement outlining a plan to cut greenhouse emissions.

Dr. Neal Lane, a physicist who is the presidential science adviser, said in
an interview that it was important not to overplay the study, which relied on a
fairly simple computer model and estimated variations in the sun's power that
were, at best, rough approximations.

But the new study largely correlates with work by Dr. Mann and others that
used different methods but reached similar conclusions.

"Scientists will debate whether this is the smoking gun," Dr. Lane said. "But
it's clear that the changes we've seen in this century are unprecedented in the
last thousand years. This argues for prudent action to stem the growth of
greenhouse gases."

Critics of the global warming theory quickly took note of the work, but
mainly to poke at its weaknesses. For example, Dr. S. Fred Singer, the president
of Science & Environmental Policy Project, a private consulting group in
Virginia, said Dr. Crowley had drawn precise conclusions from imprecise
calculations.

But other scientists defended Dr. Crowley's approach.

The initial goal in the work, Dr. Crowley said, was to gain a better
understanding of how natural influences were shaping past global temperatures,
which have included a variety of shifts ranging from a medieval warming to a
prolonged cooling from the 1600's to the 1800's that is called the Little Ice
Age.

Two of the most important factors are changes in the radiation flowing from
the sun, which is thought to follow many complex cycles, and volcanoes, which
temporarily cool the earth by lofting a veil of fine sulfate droplets that
reflect sunlight.

Dr. Crowley found that past variations in the radiance of the sun or bursts
of volcanic activity, when fed into a computer model simulating the flow of
energy to and from the earth, produced temperatures that match most of the ups
and downs of the actual climate from the year 1000 to the mid 1800's.

But that neat relationship among sunlight, eruptions and temperature broke
down completely in the 20th century, he reported. The only "forcing" that
remotely matched the jump in temperatures seen in the latter half of the century
was the rise in emissions of greenhouse gases.

Dr. Crowley himself noted that his model was rough; for example, he said,
past solar variation is only indirectly recorded, partly in traces of elements
found in old glacial ice that were made as radiation from the sun and other
stars bombarded the atmosphere, and partly in observations by Galileo and
subsequent astronomers who charted sunspot activity from 1610 on.

Even so, the good correlation between his work and other recent
reconstructions of past climates prompted a variety of climate experts to say
the study was valid. A big step forward.

A separate study, to be published tomorrow in Geophysical Research Letters,
an influential journal, largely echoes Dr. Crowley's assertion that human
actions are dominating current climate shifts. But the second study cautions
that other, natural factors could stall further warming.

One of the authors of that report, Dr. Michael E. Schlesinger, a
climatologist at the University of Illinois at Urbana-Champaign, said it was
particularly important for policy makers and the public not to assume that
temperature trends will follow a smooth course.

He said that the relationship between the oceans and the atmosphere is so
complex that alterations in it could easily cause temporary cool periods or
other unpredictable -- and possibly abrupt -- changes that could create
confusion and paralyze work to attack global warming.

A key factor hampering our ability to confidently assess the human influence
on the warming of the past century is our limited understanding of the climate
changes believed to have occurred in previous centuries. What caused the "Little
Ice Age" of the 15th to 19th centuries or the putative "Medieval Warm Period" of
earlier centuries (

1, 2)? Might not the same,
presumably natural, factors bear some responsibility for the dramatic warming of
the 20th century (3-6)? On page 270 of this issue, Crowley
(7) provides some convincing answers to these questions and makes a
compelling case for the assertion that anthropogenic greenhouse gas increases
are behind the continued warming of the globe.

Conventional approaches to understanding the factors underlying the recent
warming have involved complex numerical models of the combined ocean-atmosphere
system. Although highly suggestive of a detectable human influence on climate,
these studies have been limited by intrinsic uncertainties in comparing
model-predicted climate change patterns with the instrumental climate record. At
roughly one century, the latter is too short to allow unambiguous attribution of
changes to human influences (

Crowley's study circumvents this limitation by making use of empirical
information about longer term climate variability. The author uses an Energy
Balance Model (EBM), calibrated to exhibit a similar response to external
radiative influences as more elaborate coupled ocean-atmosphere models. This
allows an efficient investigation of forced changes in annual mean temperatures
in the Northern Hemisphere over the past millennium. The model is driven with
(admittedly uncertain) empirical estimates of the time histories of the most
relevant factors affecting the atmosphere's radiative balance (solar radiative
output, volcanic aerosol loading,

anthropogenic greenhouse gas concentrations, and industrial aerosols).
Comparison of the predicted response with independent (although also uncertain)
estimates of Northern Hemisphere annual temperature variations over the past
millennium based on proxies such as tree rings, ice cores, and corals, which
naturally record climate variations (

9, 10) (see the figure),
yields fairly close agreement (11). Of equal interest, however, is
the level of disagreement: Within estimated uncertainties, the amplitude of the
residual temperature variations not explained by the model agrees precisely with
the typical amplitude of purely random or "stochastic" climate variability
observed in coupled ocean-atmosphere models.

9) with the EBM results described by
Crowley (7). The blue-shaded region represents the approximate
uncertainty range in the empirical temperature estimates of (9).
Two extratropical warm-season Northern Hemisphere temperature reconstructions
(20, 21) are shown for comparison.

Crowley's report thus strengthens the case for a detectable human influence
on 20th century global warming by establishing that (i) much of the climate
history of the past millennium can be explained in terms of a few
well-established, physically well-constrained radiative forcings, (ii) the
dramatic warming of the 20th century can almost certainly not be explained by
the natural forcings, but instead requires the emergent anthropogenic forcings
of the 20th century, and (iii) more detailed climate models used to detect and
attribute observed patterns of climate change to anthropogenic factors
(

8) appear to capture the unforced component of climate variability with
sufficient accuracy. The last conclusion strengthens the independent conclusion
drawn from simulations using more complex models that human-induced climate
change is now detectable.

Nonetheless, Crowley's study does not explain the entire climate history of
the past millennium. The model does not, for example, reproduce the cooling of
the late 19th century that is seen both in proxy-based climate reconstructions
(

9, 10) and the early instrumental record (12); the warming, in essence, begins too soon in the model. One possible
explanation offered by Crowley is that both the reconstructions and the
instrumental record may independently underestimate the hemispheric temperatures
during this period, for example, because of sparse spatial sampling. A better
explanation, however, also noted by Crowley, is that a potentially important
surface radiative forcing not included in his simulations--land usage changes,
which affect Earth's surface albedo--may be responsible for the observed
cooling. A recent study (13) indicates that anthropogenic
large-scale land usage changes should have culminated in an annual mean cooling
of more than 0.3ºC in the 19th century. This additional anthropogenic forcing is
not only large enough to explain the discrepancy between observation and
Crowley's EBM results, it has also been implicated (14)
in another residual discrepancy, namely the observed differences between
conventional proxy-based estimates of past hemispheric temperature changes
(9, 10) and ground surface temperature estimates from borehole
profiles (15).

Crowley's study also does not explain the regional complexity of surface
temperature trends during the past millennium. There is little doubt that the
temperature anomalies associated with the Little Ice Age and the Medieval Warm
Period were far more prominent in some regions (such as Europe) than in others.
These large regional anomalies vary in amplitude, timing, and sign and thus
average out to yield more modest variations for the Northern Hemisphere on the
whole (

9, 10). In recent decades, Europe has
warmed faster than the Northern Hemisphere on the whole, whereas certain regions
in the North Atlantic have actually cooled in the face of widespread warming.
This is a result of a combination of regional temperature overprints by the
North Atlantic Oscillation (NAO) and related, but distinct, patterns of
multidecadal variability associated with the thermohaline circulation of the
North Atlantic (16, 17).

It is quite reasonable to assume that similar factors were associated with
the pronounced temperature changes in Europe in past centuries that accompanied
more modest hemispheric-wide temperature changes. Keigwin and Pickart (

18)
have shown evidence that a heterogeneous temperature pattern in the North
Atlantic region consistent with the NAO coincided with the European Medieval
Warm Period and Little Ice Age. There is evidence that the aforementioned
multidecadal variations in the North Atlantic can couple to variations in solar
radiative output that occur on similar time scales (19).

Could a similar mode of North Atlantic variability resonate with solar
radiative variations at millennial time scales, imprinting a regional pattern of
enhanced anomalies on top of the more modest hemispheric-scale warming that
Crowley's study attributes in part to solar forcing at these time scales? Only
further, more detailed modeling studies and expanded networks of paleoclimate
indicators will further elucidate the spatial and temporal patterns of climate
change in past centuries.

The author is in the Department of Environmental Sciences, University of
Virginia, Charlottesville, VA 22902, USA. E-mail: mann@virginia.edu

Science, 14 July, 2000 v. 289

Recent reconstructions of Northern Hemisphere temperatures and climate
forcing over the past 1000 years allow the warmingof the 20th
century to be placed within a historical context andvarious
mechanisms of climate change to be tested. Comparisonsof
observations with simulations from an energy balance climatemodel
indicate that as much as 41 to 64% of preanthropogenic (pre-1850)decadal-scale temperature variations was due to changes in solarirradiance and volcanism. Removal of the forced response fromreconstructed temperature time series yields residuals that showsimilar variability to those of control runs of coupled models,thereby lending support to the models' value as estimates of
low-frequencyvariability in the climate system. Removal of all
forcing exceptgreenhouse gases from the ~1000-year time series
results in aresidual with a very large late-20th-century warming
that closelyagrees with the response predicted from greenhouse gas
forcing.The combination of a unique level of temperature increase in
thelate 20th century and improved constraints on the role of
naturalvariability provides further evidence that the greenhouse
effecthas already established itself above the level of natural
variabilityin the climate system. A 21st-century global warming
projectionfar exceeds the natural variability of the past 1000 years
andis greater than the best estimate of global temperature
changefor the last interglacial.

1) suggestthat
anthropogenic changes, particularly greenhouse gas (GHG)increases,
are probably responsible for this climate change. However,there are
a number of persistent questions with respect to theseconclusions
that involve uncertainties in the level of low-frequencyunforced
variability in the climate system (2) and whetherfactors
such as an increase in solar irradiance or a reductionin volcanism
might account for a substantial amount of the observed20th-century
warming (1, 3-10).Although many
studies have addressed this issue from the paleoclimateperspective
of the past few centuries (3-10),robust conclusions
have been hampered by inadequate lengths ofthe time series being
evaluated. Here I show that the agreementbetween model results and
observations for the past 1000 yearsis sufficiently compelling to
allow one to conclude that naturalvariability plays only a
subsidiary role in the 20th-century warmingand that the most
parsimonious explanation for most of the warmingis that it is due to
the anthropogenic increase in GHG.

Data

The data used in this study include physically based reconstructions of
Northern Hemisphere temperatures and indices of volcanism,solar
variability, and changes in GHGs and tropospheric aerosols.

Northern Hemisphere temperatures. Four indices of millennial Northern
Hemisphere temperature have been produced over the past3 years
(

11-14). The analysishere uses the mean annual
temperature reconstructions of Mannet al. (11)
and of Crowley and Lowery (CL) (12),because the energy
balance model used in this study calculatesonly this term [the other
records (13, 14)are estimates of
warm-season temperature at mid-high latitudes].The Mann et
al. reconstruction was determined (8) byfirst regressing an
empirical orthogonal function analysis of20th-century mean annual
temperatures against various proxy indices(such as tree rings,
corals, and ice cores). Past changes in temperatureare estimated
from variations in the proxy data (15).The Mann et
al. reconstruction has a varying number of recordsper unit of
time (although the number in the earlier part of therecord is still
greater than in CL). The CL reconstruction isa more heterogeneous
mix of data than the Mann et al. reconstruction,but the
number of records is nearly constant in time. It is asimple
composite of Northern Hemisphere climate records and wasscaled
(12) to temperature using the instrumental record(16) in the overlap interval 1860-1965. The instrumentalrecord was substituted for the proxy record after 1860 for tworeasons: (i) there were too few proxy data in the CL time seriesafter 1965 to reconstruct temperatures for this interval, and(ii) the original CL reconstruction indicated a "warming" overthe interval 1885-1925 that is at variance with the instrumentalrecord. This difference has been attributed (11, 17)to an early CO2 fertilization effect on tree
growth. The significanceof this decision will be further discussed
below; model-data correlationspresented in the study include both
the original proxy recordand the substituted instrumental time
series.

Despite the different number and types of data and different methods of
estimating temperatures, comparison of the decadallysmoothed
variations in each reconstruction (

Fig. 1) indicatesgood
agreement (r = 0.73 for 11-point smoothed correlations overthe preanthropogenic interval 1005-1850, with P < 0.01).
Bothrecords [and the Jones et al. (13)
and Briffa (14)reconstructions] show the "Medieval Warm
Period" in the interval~1000-1300, a transition interval from about
1300-1580, the 17th-centurycold period, the 18th-century recovery,
and a cold period in theearly 19th century. Even many of the
decadal-scale variationsin the Medieval Warm Period are reproducible
(12),and both reconstructions [and (13,
14)]indicate that peak Northern Hemisphere warmth during the
MiddleAges was less than or at most comparable to the
mid-20th-centurywarm period (~1935-1965). This result occurs because
Medievaltemperature peaks were not synchronous in all records
(12).The two temperature reconstructions also agree
closely in estimatingan ~0.4°C warming between the 17th-century and
the mid-20th-centurywarm period (18).

11) and CL (12).
The latter record has been spliced into the 11-point smoothed instrumental
record (16) in the interval in which they overlap. CL2 refers to a new
splice that gives a slightly better fit than the original (12).
The autocorrelation of the raw Mann et al. time series has been used to
adjust (adj) the standard deviation units for the reduction in variance on
decadal scales. [View Larger Version of this Image (34K GIF
file)]

Volcanic forcing. There is increasing evidence (

3,
7-10) that pulses of volcanism significantlycontributed to decadal-scale climate variability in the LittleIce Age. Although some earlier studies (9, 10)of forced climate change back to 1400 used a composite ice
coreindex of volcanism (19), which has a different
numberof records per unit of time, the present study primarily
usestwo long ice core records from Crete (20)
and the GreenlandIce Sheet Project 2 (GISP2) (21)
on Greenland, witha small augmentation from a study of large
eruptions recordedin ice cores from both Greenland and Antarctica
(22).This approach avoids the potential for biasing
model results versustime because of changes in the number of
records. Because SouthernHemisphere volcanism north of 20°S
influences Northern Hemispheretemperatures, the ice core volcano
census samples records downto this latitude. The volcanism record is
based on electricalconductivity (20) or sulfate
measurements (21),and a catalog of
volcanic eruptions (23) was used toremove
local eruptions (24) and identify possible
candidateeruptions in order to weight the forcing according to
latitude.Eruptions of unknown origin were assigned a high-latitude
originunless they also occurred in Antarctic ice core records
(22).

The relative amplitude of volcanic peaks was converted to sulfate
concentration by first scaling the peaks to the 1883 Krakataupeak in
the ice cores. Although earlier studies (

9,10)
linearly converted these concentration changesto radiative forcing
changes, subsequent comparison (25)of the very large
1259 eruption [eight times the concentrationof sulfate in ice cores
from Krakatau and three times the sizeof the Tambora (1815) eruption
(21)] with reconstructedtemperatures (11-14) failed tosubstantiate a response commensurate with a
linearly scaled predictionof an enormous perturbation of ~25
W/m2 (26). Calculations (27)
suggest that forstratospheric sulfate loadings greater than about 15
megatons(Mt), increasing the amount of sulfate increases the size of
aerosolsthrough coagulation. Because the amount of scattered
radiationis proportional to the cross-sectional area, and hence to
the2/3 power of volume (or mass), ice core concentrations
estimatedas >15 Mt were scaled by this amount (25).
Aerosoloptical depth was converted to changes in downward shortwave
radiativeforcing at the tropopause, using the relationship discussed
inSato et al. (28). There is significant agreement
(29)between the 1000-year-long volcano time series
and the concentration-modifiedRobock and Free (19)
times series (Fig.
2A).Both proxy records show the
general trends estimated from ground-basedobservations of aerosol
optical depth (28): the pulseof
eruptions in the early 20th century and the nearly 40-yearquiescent
period of volcanism between about 1920-1960. Becausevolcano peaks
are more difficult to determine in the expandedfirn layer of
snow/ice cores, updated estimates of Northern Hemisphereradiative
forcing from Sato et al. were used to extend proxy timeseries
from 1960 to 1998.

Solar forcing. There has been much discussion about the effect of
solar variability on decadal-to-centennial-scale climates(3,
6, 8-10). Anupdated version of a reconstruction by Lean
et al. (5)that spans the interval 1610-1998 was used to
evaluate this mechanism[for reference, Free and Robock (10)
obtained comparablesolar-temperature correlations for the interval
1700-1980 usingthe Lean et al. and alternate Hoyt and
Schatten (4)solar reconstructions]. The Lean et al.
time series has been extendedto 1000 by splicing in different
estimates of solar variabilitybased on cosmogenic isotopes. These
estimates were derived fromice core measurements (30)
of 10Be, residual 14C from tree ring records (31),
and an estimate of 14C from 10Be fluctuations (30).
The justification for includingthe latter index is that neither of
the first two splices yieldsa Medieval solar maximum comparable to
that of the present. Becauseof concerns about biasing results too
much by the latter period,which has much more information than the
former, the Bard 14C calculation was included so as to obtain a
greater spread ofpotential solar variations and to allow testing of
suggestions(32) that solar irradiance increases
could explainthe Medieval warming.

Once the splices were obtained, the records were adjusted to yield the
potential ~0.25% change in solar irradiance on longertime scales
(

33). Because two of the solar proxiesindicate that
minimum solar activity occurred in the 14th century,the 0.25% range
was set from that time to the present rather thanfrom the 17th
century, as was done by Lean et al. [the adjustmentis very
small for the different solar indices in the 14th century(~0.05
W/m2)]. The 20th-century increase in estimated net radiative
forcingfrom low-frequency solar variability is about 10 to 30%
greaterthan estimated from an independent method (34).
Anexample of one of the splices is illustrated in Fig. 2B,and the three composites (Fig. 2C) show the
pattern ofpotential solar variability changes used in this
study.

Anthropogenic forcing. The standard equivalent radiative forcing for
CO2 and other well-mixed trace gases (methane, nitrousoxides, and chlorofluorocarbons) is used after 1850 (

Fig. 2D).Pre-1850 CO2 variations, including the small
minimum from about1600-1800, are from Etheridge et al.
(35). Radiativeforcing effects were computed based
on updated radiative transfercalculations (36).
The well-constrained change in GHGforcing since the middle of the
last century is about four timeslarger than the potential changes in
solar variability based onthe reconstructions of Lean et al.
(5) and Lockwoodand Stamper (34).

37), with the
Northern-to-Southern-Hemisphere ratiobeing in the range of 3 to 4
(38). Because there isan approximate offset in the
radiative effects of stratosphericand tropospheric ozone (37),
and its total net forcingis on the order of +0.2 W/m2
(37) and is applicable only to the late 20th century,this GHG was not further considered. Other anthropogenic forcingwas not included because evaluations by the IntergovernmentalPanel for Climate Change (IPCC) (37) indicate thatthe
confidence in these estimates is very low.

Model

A linear upwelling/diffusion energy balance model (EBM) was used to calculate
the mean annual temperature response to estimatedforcing changes.
This model (

39) calculates the temperatureof a vertically
averaged mixed-layer ocean/atmosphere that isa function of forcing
changes and radiative damping. The mixedlayer is coupled to the deep
ocean with an upwelling/diffusionequation in order to allow for heat
storage in the ocean interior.The radiative damping term can be
adjusted to embrace the standardrange of IPCC sensitivities for a
doubling of CO2. The EBM issimilar to that used in many
IPCC assessments (40)and has been
validated (39) against both the Wigley-RaperEBM (40)
and two different coupled ocean-atmospheregeneral circulation model
(GCM) simulations (41). Allforcings for the
model runs were set to an equilibrium sensitivityof 2°C for a
doubling of CO2. This is on the lower end of theIPCC
range (42) of 1.5° to 4.5°C for a doubling ofCO2 and is slightly less than the IPCC "best guess"
sensitivityof 2.5°C [the inclusion of solar variability in model
calculationscan decrease the best fit sensitivity (9)].
For boththe solar and volcanism runs, the calculated temperature
responseis based on net radiative forcing after adjusting for the
30%albedo of the Earth-atmosphere system over visible
wavelengths.

Results

The modeled responses to individual forcing terms (

Fig. 3A) indicate that
the post-1850 GHG and tropospheric aerosolchanges are similar to
those discussed in IPCC (42).CO2 temperature variations are very small for the
preanthropogenicinterval, although there is a 0.05°C decrease in the
17th and18th centuries that reflects the CO2 decrease of
~6 parts permillion in the original ice core record (35).
Solarvariations are on the order of 0.2°C, and volcanism causes
largecooling (43) in the Little Ice Age (3-7,9, 10). Averaged over the
entire preanthropogenicinterval (Table 1), 22 to 23% of
the decadal-scale variancecan be explained by volcanism (P
0.01). However, over the interval1400-1850, the volcanic
contribution increases to 41 to 49% (P 0.01), thereby
indicating a very important role for volcanismduring the Little Ice
Age.

Fig. 3. (A) Model response to different forcings,
calculated at a sensitivity of 2.0°C for a doubling of CO2.
(B) Example of the combined effect of volcanism and solar variability
(with 11-point smoothing), using the Bard et al. (30)
14C index.

Table 1. Correlations of
volcanism (volc.) and solar variability (sol.) for the preanthropogenic
interval, with percent variance shown in parentheses. The different solar
time series reflect the three different solar indices used in this study.
The Mann et al. time series (11) has been
smoothed with an 11-point filter. CL was smoothed in the original analysis
(12). Different abbreviations for solar forcing
refer to the different indices discussed in the text: 10Be and
14C calculations are from Bard et al. (30); 14C residuals are from Stuiver et al.
(31).

Volc. vs. Mann et al. (1000-1850)

0.48 (23%)

Volc. vs. CL (1000-1850)

0.47 (22%)

Volc. vs. Mann et al. (1400-1850)

0.70 (49%)

Volc. vs. CL (1400-1850)

0.64 (41%)

Sol (10Be) vs. Mann et al.

0.45 (20%)

Sol (14C Bard) vs. Mann et al.

0.56 (31%)

Sol (14C Stuiver) vs. Mann et al.

0.37 (14%)

Sol (10Be) vs. CL

0.42 (18%)

Sol (14C Bard) vs. CL

0.67 (45%)

Sol (14C Stuiver) vs. CL

0.30 ( 9%)

The sun-climate correlations for the interval 1000-1850 vary substantially by
choice of solar index (

Table
1), withexplained variance
ranging from as low as 9% (P 0.01) for the 14C residual
index (31) to as high as 45% (P 0.01)for the
Bard et al. (30) 14C solar index,
which reconstructs a Medieval solar warming comparableto the present
century but only about 0.1°C greater than predictedby the other
solar indices (Fig.
3A). The large rangein
correlations for the solar records emphasizes the need to determinemore precisely the relative magnitude of the real Medieval solarwarming peak.

The joint effects of solar variability and volcanism (

Fig. 3B) indicate that the combination of these effects couldhave
contributed 0.15° to 0.2°C to the temperature increase (Fig. 1)from about 1905-1955, but only about one-quarter to the
total20th-century warming. The combined warmth produced by solar
variabilityand volcanism in the 1950s is similar in magnitude but
shorterin duration than the warmth simulated by these mechanisms in
theMiddle Ages. The variations in the past few decades
resultingfrom the combination of solar variability and volcanism is
0.2°Cless than the 1955 peak.

Combining all forcing (solar, volcanism, GHG, and tropospheric aerosols)
results in some striking correspondences betweenthe model and the
data over the preanthropogenic interval (

Fig. 4).Eleven-point
smoothed correlations (44) for the preanthropogenicinterval (Table
2) indicate that 41 to 64% of the
totalvariance is forced (P < 0.01). The highest
correlations are obtainedfor the CL time series, which has slightly
more Medieval warmththan the Mann et al. reconstruction, and
for the forcing timeseries that includes the largest solar estimate
of Medieval warmth.Forced variability explains 41 to 59% of the
variance (P 0.01)over the entire length of the records.
Although simulated temperaturesagree with observations in the late
20th century, simulationsexceed observations by ~0.1° to 0.15°C over
the intervals 1850-1885and 1925-1975, with a larger discrepancy
between ~1885-1925 thatreaches a maximum offset of ~0.3°C from
~1900-1920. However, decadal-scalepatterns of warming and cooling
are still simulated well in theseoffset intervals. A sensitivity
test (45) comparingforcing time series with and without
solar variability indicatesthat changes caused by volcanism and
CO2 are responsible for thesimulated temperature increase
from the mid- to late 19th centuryto the early 20th century, thereby
eliminating uncertainties insolar forcing as the explanation for the
temperature differencesbetween the model and the data. Also shown in
Fig. 4Ais the CL reconstruction with the "anomalous"
warm interval (~1885-1925)discussed above. For this reconstruction,
55 to 69% of the variancefrom 1005-1993 can be explained by the
model (P 0.01).

Fig. 4. Comparison of model response (blue) using all
forcing terms (with a sensitivity of 2.0°C) against (A) the CL (12)
data set spliced into the 11-point smoothed Jones et al. (16)
Northern Hemisphere instrumental record, with rescaling as discussed in the text
and in the Fig. 1 caption; and (B) the smoothed Mann et al.
(11) reconstruction. Both panels include the Jones et
al. instrumental record for reference. To illustrate variations in the
modeled response, the 14C calculation from Bard et al.
(30) has been used in (A) and the 10Be estimates
from (30) have been used in (B).

Table 2. Correlations between
model runs with combined forcing and the Mann et al. (11) and CL (12) time series. Correlations
have been subdivided into the following three categories: Top set:
Correlations for all the preanthropogenic interval 1005-1850 of model
response to combined forcing ("All") with different solar indices
(Table 1) and the 11-point smoothed Mann et al. time
series and CL2 record spliced into the 11-point smoothed Jones et
al. (16) time series. Middle set:
Correlations over the entire interval analyzed. Bottom set: Correlations
and variance explained for the interval 1005-1993 using the original CL2
reconstruction from 1005-1965, with the smoothed Jones et al.
(16) record added from 1965-1993.

Summary of pre-1850 correlations, with variance shown in parentheses

All 10Be (solar) vs. Mann (sm11)

0.64 (41%)

All 14C Brd (solar) vs. Mann (sm11)

0.68 (46%)

All 14C Stv (solar) vs. Mann (sm11)

0.65 (42%)

All 10Be (solar) vs. CL2.Jns11

0.69 (48%)

All 14C Brd (solar) vs. CL2.Jns11

0.80 (64%)

All 14C Stv (solar) vs. CL2.Jns11

0.68 (47%)

Summary of correlations for 1005-1993, with variance shown in
parentheses

All 10Be (solar) vs. Mann (sm11)

0.68 (46%)

All 14C Brd (solar) vs. Mann (sm11)

0.73 (53%)

All 14C Stv (solar) vs. Mann (sm11)

0.67 (45%)

All 10Be (solar) vs. CL2.Jns11

0.66 (43%)

All 14C Brd (solar) vs. CL2.Jns11

0.77 (59%)

All 14C Stv (solar) vs. CL2.Jns11

0.64 (41%)

Summary of correlations for 1005-1993 against unfiltered CL time
series, with 11-point smoothed Jones et al. (16) record spliced in from
1965-1993

All 10Be (solar) vs. CL2.Jns 11

0.75 (57%)

All 14C Brd (solar) vs. CL2.Jns11

0.83 (69%)

All 14C Stv (solar) vs. CL2.Jns11

0.74 (54%)

Another means of evaluating the role of forced variability is to determine
residuals by subtracting the different model timeseries from the two
paleo time series over the preanthropogenicinterval (

Fig. 5A). The trend lines for three of theseresiduals are virtually
zero, and there is only about a ±0.1°Ctrend for the other three
residuals. Because the pre-1850 residualsrepresent an estimate of
the unforced variability in the climatesystem, it is of interest to
compare the smoothed residuals withsmoothed estimates of unforced
variability in the climate systemfrom control runs of coupled
ocean-atmosphere models. There issignificant agreement (Fig. 5B and Table
3)between the smoothed standard
deviations of the GCMs (46)and paleo residuals
(47). These results support a basicassumption in
optimal detection studies (1) and previousconclusions (48) that the late-20th-century
warmingcannot be explained by unforced variability in the
ocean-atmospheresystem. However, a combination of GHG, natural
forcing, and ocean-atmospherevariability could have contributed to
th